In an inkjet-type image forming apparatus (hereinafter, referred to as an inkjet printer), images or letters/characters are recorded on a recording sheet in a well-known manner. For example, a recording sheet is displaced or conveyed while ink is not ejected onto the recording sheet from a recording head. Ink is ejected onto the recording sheet from the recording head while the recording head reciprocates to scan while the recording sheet remains at rest. That is, in the inkjet printer, an image is formed on a recording sheet by alternately conveying, intermittently, a recording sheet and recording of an image on the recording sheet.
As described above, in the inkjet printer, an image is formed by which a recording sheet is intermittently conveyed. Therefore, if ink positions where ink droplets adhere to or land on the recording sheet deviate from their expected landing positions because of variations in a conveying amount of the recording sheet among each conveyance white streaks (lines) or dark streaks (lines) extending in a direction parallel to a scanning direction of the recording head (a main scanning direction) tend to appear on the recording sheet (hereinafter, such a recording failure is referred to as “banding”).
As is clear from the above description, the conveying amount of the recording sheet needs to be accurately controlled in order to avoid an occurrence of banding. However, thicknesses and surface conditions of recording sheets vary among types. Thus, it is difficult to accurately convey all types of recording sheets.
In one example, two sensors are provided at a predetermined distance therebetween in a recording sheet conveying direction. A rotation amount of a conveyor roller is corrected in accordance with a ratio between a rotation amount (a drive amount) of a conveyor roller while a recording sheet to be used is being conveyed between the sensors and a rotation amount of a conveyor roller while a reference recording sheet is being conveyed between the sensors.
Generally, in a conveying path extending from a sheet feed tray to an image forming portion (a recording head), at least a sheet feed roller and a conveyor roller are provided. The sheet feed roller conveys a recording sheet to the image forming portion by rotating while contacting the recording sheet placed in the sheet feed tray. The conveyor roller is disposed downstream of the sheet feed roller in the conveying direction to apply a conveying force onto the recording sheet by rotating while contacting the recording sheet.
Generally, a peripheral speed of the sheet feed roller is smaller than that of the conveyor roller. Therefore, while a recording sheet being conveyed is in contact with the conveyor roller at its leading edge in the conveying direction and is in contact with the sheet feed roller at its trailing edge in the conveying direction, the recording sheet may experience a backward force resulting from the relatively slower rotation speed of the sheet feed roller. Thus, a conveying load on the conveyor roller becomes larger (i.e., a greater conveying force is needed to convey the recording sheet forward).
When the conveying load on the conveyor roller reaches a load threshold, the recording sheet slips from the conveyor roller. Thus, in some instances, the conveying amount of the recording sheet cannot be accurately controlled, thereby causing banding on the recording sheet.